Bacteria of the genus Yersinia are responsible for a variety of human diseases. Y. pestis causes the infamous disease Plague, which regained prominence due to its potential use by bioterrorists. In contrast, Y. pseudotuberculosis and Y. enterocolitica cause primarily gastrointestinal disease. However, despite the differences in disease symptoms, these pathogenic Yersinia species are closely related and share several common virulence determinants. Yersinia studies have provided fundamental insights into bacterial pathogenesis, including the first example of the widespread type three secretion system (T3SS). A critical component of all T3SSs is an outer membrane pore-forming protein known as a secretin. However, secretin production can cause bacterial cell envelope stress. This is lethal to Y. enterocolitica unless a critical stress response known as the phage shock protein (Psp) system is functional. As a result, the Psp system of Y. enterocolitica is essential for virulence. Furthermore, the Psp system is also important for virulence and additional health-related processes in other bacteria. Our studies of the Psp system have identified its core components and defined their roles in regulation and stress tolerance. Some of our future work will focus on the two critical aspects of the Psp system required for Ye virulence: how is it activated and how is T3SS-induced cell death prevented? We have also identified a highly conserved gene (YE0566) that can both activate psp gene expression and suppress secretin-sensitivity in a psp null strain. This gene might direct the production of a 53 amino acid cytoplasmic membrane protein when cells are growing rapidly or an RpoS- induced non-coding RNA when cells enter stationary phase. We want to understand how YE0566 induces the Psp system and how it suppresses secretin-sensitivity when the Psp system is absent. To address all of our goals we propose to: (1) Investigate how an inducing signal is detected and transduced by the Psp system; (2) Analyze how the PspB and PspC proteins prevent secretins from permeabilizing the cytoplasmic membrane; (3) Investigate the regulation and function of YE0566. These studies also have broad significance beyond Y. enterocolitica because secretin-containing systems critical for virulence, the Psp system, and YE0566 homologues are widespread in medically important bacteria.